In recent years, all digital phase locked loop (ADPLL) circuits, which are phase locked loop (PLL) circuits where all the control signals are digitalized, have been used as local oscillators in radio communication apparatuses and the like. The ADPLL circuits use digital circuits in place of analog circuits, and thus can save space and electric power consumption.
An ADPLL circuit includes a digital loop filter, a digitally-controlled oscillator (DCO), and a phase data generator (PDG). The digitally-controlled oscillator outputs an oscillation signal with an oscillation frequency corresponding to an oscillator control value. The phase data generator outputs a phase difference between a reference signal and the oscillation signal. The results obtained by subtracting the phase difference from set frequency data is filtered by the digital loop filter. The filtered signal is multiplied by a coefficient Kosc, and the product thus obtained is set as the oscillator control value.
The digitally-controlled oscillator is an analog circuit. Accordingly, the amount of change in the oscillation frequency per unit oscillator control value fluctuates depending on the process, source voltage, and temperature (PVT). When the amount of change in the oscillation frequency per unit oscillator control value fluctuates, the already-set coefficient Kosc is no longer optimal. Hence, the phase noise characteristics of the oscillation signal outputted by the local oscillator in the stable state (locked-up state) also fluctuate depending on the PVT. To put it differently, phase noise characteristics stable and independent of the PVT cannot be obtained.
To address this problem, the following technique is known. Firstly, the oscillator control value of the digitally-controlled oscillator is acquired with the ADPLL circuit made stable at a certain oscillation frequency. Then, the oscillator control value of the digitally-controlled oscillator is acquired with the ADPLL circuit made stable at a different oscillation frequency. After that, an accurate amount of change in the oscillation frequency per unit oscillator control value under the current conditions of the PVT is calculated from the difference between the two oscillation frequencies and the difference between the two oscillator control values. Thus, phase noise characteristics stable and independent of the PVT can be obtained by adjusting the coefficient Kosc appropriately on the basis of the value thus calculated.
This technique, however, requires a long time to adjust the coefficient Kosc because the ADPLL circuit is locked up twice.